You can define the following configuration options globally or on the CaptchaType itself:

width: the width of the captcha image (default=120)

height: the height of the captcha image (default=40)

disabled: disable globally the CAPTCHAs (can be useful in dev environment), it will
still appear but won't be editable and won't be checked

length: the length of the captcha (number of chars, default 5)

quality: jpeg quality of captchas (default=30)

charset: the charset used for code generation (default=abcdefhjkmnprstuvwxyz23456789)

font: the font to use (default is random among some pre-provided fonts), this should be an absolute path

keep_value: the value will be the same until the form is posted, even if the page is refreshed (default=true)

as_file: if set to true an image file will be created instead of embedding to please IE6/7 (default=false)

as_url: if set to true, a URL will be used in the image tag and will handle captcha generation. This can be used in a multiple-server environment and support IE6/7 (default=false)

invalid_message: error message displayed when an non-matching code is submitted (default="Bad code value", see the translation section for more information)

bypass_code: code that will always validate the captcha (default=null)

whitelist_key: the session key to use for keep the session keys that can be used for captcha storage, when using as_url (default=captcha_whitelist_key)

reload: adds a link to reload the code

humanity: number of extra forms that the user can submit after a correct validation, if set to a value different of 0, only 1 over (1+humanity) forms will contain a CAPTCHA (default=0, i.e each form will contain the CAPTCHA)

distortion: enable or disable the distortion on the image (default=true, enabled)

max_front_lines, max_behind_lines: the maximum number of lines to draw on top/behind the image. 0 will draw no lines; null will use the default algorithm (the
number of lines depends on the size of the image). (default=null)

background_color: sets the background color, if you want to force it, this should be an array of r,g &b, for instance [255, 255, 255] will force the background to be white